Student Peer Mentoring Programs

I am looking to implement a student-to-student peer mentoring program in a high school setting, and welcome any advice/suggestions/guidance.
Here's some background info: The school is a specialized health and science technology high school in an urban district where students must apply and go through a selection process in order to enroll. In the past couple of years enrollment has increased, but so has the number of adolescents who struggle through the first semester and end up returning to the regular h.s. So after organizing some focus groups with students, we have decided to pilot a program where juniors would "mentor" incoming freshman. Any one running a similar program in their school setting??